This invention relates generally to spatial light modulators including those which use liquid crystal on silicon (LCOS), grating light valves and digital micro mirror devices (DMMD), as examples.
In spatial light modulators, light modulation is achieved by controlling the electro-optic modulation of individual pixel elements. An inter-pixel gap between the pixel elements is generally assumed to be non-reflective. Therefore the gap is assumed not to interfere with modulation mechanism of the individual pixel elements.
However, the optical properties of the inter-pixel gap determine the optical coupling to the substrate and the contrast of the spatial light modulator. For example, the pixel elements may in some cases be formed of a plurality of pixel plates which are spaced from one another. It is generally believed that the layer under these pixel plates is not significantly reflective. This layer under the pixel plates, which in some designs may be the metal layer, may include an anti-reflective coating of titanium nitride (TiN) and a metallic absorbing layer. See, E. G. Colgan and M. Uda, On-Chip Metallization Layer for Reflective Light Valves, IBM Journal of Research and Development, 339-335, Vol. 42, May/July 1998. This report states that both the metal and the anti-reflective layers are made sufficiently thick that they are essentially optically opaque. Light incident between the pixel plates requires multiple reflections between the top of the anti-reflective layer and the bottom of the material layer to reach the silicon substrate. However, the report states that the titanium nitrate anti-reflective layer actually has a reflectivity of about twenty percent for blue light and sixty-five percent for red light.
Reflectance from light which passes through the inter-pixel gap affects contrast. Contrast is reduced because a dark pixel is not completely dark. The extraneous light is reflected from all four sides of the pixel gap. The white-dark transition and dark-dark transitions in the image have reduced visibility.
Thus, there is a continuing need for better ways to reduce the reflectance from light which passes between pixel elements.